1. Field of the Invention
The present invention concerns a fifth generation x-ray computed tomography (CT) system of the type having at least one electron beam generator that generates at least one electron beam, an anode ring or partial anode ring arranged concentrically around a system axis, from which x-ray radiation is generated at a number of focus positions by striking said anode ring or partial anode ring with at least one electron beam, a detector ring or partial detector ring arranged concentrically around the system axis, with a number of detector elements forming at least one detector line to detect the impinging x-ray radiation; and a rotatable support frame to accommodate filter and collimator elements. The invention also concerns a method to control such an x-ray computed tomography system.
2. Description of the Prior Art
Fifth generation CT systems are generally known. These are CT systems with a stationary, annular anode which is arranged around the system axis, wherein x-ray radiation is generated (emitted) at a number of positions on the stationary anode in various ways. Such a CT also has a (likewise stationary) annular detector. Rotating parts are hereby largely foregone, so a relatively high scan rate can be achieved. For example, an electron emitter with corresponding deflection and concentration devices is used to generate x-ray radiation, with which devices an electron beam can be directed at desired positions of the anode ring where x-ray radiation is emitted from a focus created at those positions. U.S. Pat. Nos. 4,158,142; 4,352,021; 4,521,900 and 4,521,901 disclose examples of such “electron beam” CT systems.
Other known variants of the fifth generation CT systems use a laser in order to excite electron emission at an arbitrary point on a cathode ring arranged opposite the anode ring. The emitted electrons generate x-ray radiation on the opposite anode at the focus arising there. The document U.S. Pat. No. 4,606,061 discloses such an embodiment.
A CT system with stationary anode ring and stationary detector ring is known from DE 40 15 105 C3 and DE 40 15 180, in which a cathode ring is divided into a number of segments that can be individually activated. By activating the individual cathode segments, x-ray sources distributed along the circumference of the anode ring can be activated in a targeted manner, with which x-ray sources a circular scan of a patient is enabled.
These known CT systems have the advantage that neither an x-ray tube nor a detector (as it is typical in the third generation CT systems) must be rotated at high speed on a gantry around a system axis, so many problems are avoided that arise due to the high centrifugal forces that occur, as well as due to the necessity to transmit high voltages, high currents and large data sets via slip rings. Due to the very quickly adjustable focus, however, new problems occur with regard to optical beam filtering and collimation as well as with regard to optimal scatter radiation suppression at the detector. Optimal filtering, collimation and scatter radiation suppression requires filters and collimators that are essentially fashioned symmetrical to an imaginary axis (Focus-System Axis), and thus are designed to be variable relative to the circumferential direction. Simple filters that have a rotationally symmetrical design over the entire extent are less suitable.
One possibility for better radiation filtering and collimation of the x-ray radiation is described in DE 10 2004 061 347 B3. This document discloses a fifth generation CT system in which a focus-side filter (including a bowtie filter) and a detector-side collimator are mounted on a rotating ring, wherein both the filter and the opposite collimator are rotated such that they are respectively positioned at the focus position (here activated by a laser).
This embodiment of a CT system has the disadvantage exists that the running speed of the focus must correspond to the rotation speed of the support frame on which the focus-side filter and the detector-side collimator are mounted. The possibilities of this CT with regard to variable focus positioning are therefore severely limited, and the revolution speed of the focus is also limited by the maximum rotation speed of the support frame.